This invention relates to key telephone systems and more particularly to a paging amplifier employed in such systems.
Similar audio paging amplifiers that are used for key telephone systems employ either discrete component circuitry or some form of hybrid device. Some paging amplifiers employ built-in power sources. The paging amplifiers that are not self powered require an external power source, generally the available key telephone system power supply. Since these paging amplifiers are generally negative ground positive voltage circuits, and the key telephone system employs a positive ground negative voltage power supply, an input transformer is required to isolate the audio input signal's ground reference from the rest of the circuit.
Paging amplifiers with built-in power supplies are cost penalized compared with units that can derive their operating power from an existing key telephone system power supply. Paging amplifiers that employ input isolation transformers are sensitive to hum pickup, because of the high impedance low signal level requirements of such transformers. In addition, input transformers are generally expensive components. Many paging amplifiers provide insufficient or marginally adequate output short circuit protection, thereby decreasing reliability. Multiple operation of these prior art paging amplifiers for increased power is either impractical or technically questionable due to the parallel connection of the amplifier outputs.